A World Health Organization initiative to measure urban pollution levels around the world has found that New Delhi has the most polluted air, while the results for Beijing are as unclear as the skies there.
The study of 1,600 cities found that air pollution has worsened since a smaller survey conducted in 2011, particularly in poorer countries, leaving urban residents at a higher risk of cancer, stroke, and heart disease.
The WHO stated last month that air pollution caused approximately 7 million deaths in 2012, making it the world's largest environmental health risk.
Among the 20 cities with the most polluted air, 13 are in India, with New Delhi, Patna, Gwalior, and Raipur ranking in the top four. The annual average PM2.5 level in New Delhi is 153 micrograms per cubic meter.
Beijing, notorious for its smog, has a PM2.5 index of 56, which is just over one-third of New Delhi's pollution index, ranking it 77th. Some residents from English-speaking countries jokingly refer to Beijing as "Greyjing."
WHO experts stated that the data for China was taken from 2010, which was the most recent year for which data was provided by China. The Beijing municipal government began publishing real-time PM2.5 data in January 2012.
According to Greenpeace, one year after the data began to be published, Beijing's air quality fell to its worst level.
The Beijing municipal government stated that the annual average PM2.5 concentration in Beijing in 2013 was 89.5 micrograms per cubic meter, exceeding the standard by 156%.
Thirty-two cities reported a PM2.5 index of less than 5. Two-thirds of these are Canadian cities, including Vancouver, as well as Hafnarfjörður in Iceland, and seven are U.S. cities.
WHO experts insisted that this survey was not intended to name and shame cities with the most polluted air, as the cities participating in the survey provided information voluntarily in an effort to help them improve themselves.
Maria Neira, Director of the Department of Public Health and Environment at the WHO, said the purpose of the survey was to "challenge" cities, and she believes the results will help these cities be more transparent about the issue of air pollution.
She dismissed claims that China might be cheating, stating that collecting air pollution data has become increasingly complex.
She said: "We are having a lot of discussions with China to bring the issue of air pollution to the table. Our Director-General, Dr. Margaret Chan, has announced that China is one of the countries with serious air pollution problems. We will continue our discussions on this issue to ensure that relevant measures are in place to reduce air pollution."
